1. Field of the Invention
The present invention is directed to a process and apparatus for making solar and semiconductor grade silicon by thermal reaction of a suitable precursor gas composition. More particularly, the present invention is directed to a process and apparatus for continuous production of solar and semiconductor grade silicon in the liquid phase, by thermal decomposition of a suitable precursor gas, such as silane.
2. Description of the Prior Art
As is well known, highly pure elemental silicon properly doped with minute quantities of suitable doping agents, is the most widely used semiconductor and solar cell material. In view of the recent trend of increasing reliance on solar energy, there exists a significant demand for solar cell grade silicon at a reasonable cost. In fact, the present unavailability of solar cell grade silicon at a reasonable cost represents the principal factor which presently still renders solar cells too expensive for large-scale electrical power generation.
Solar cell or semiconductor grade (hereinafter solar grade) silicon is usually manufactured in a two-step process. First, solid silicon compounds abundantly available from the Earth's crust (such as SiO.sub.2) are converted into gaseous or low boiling liquid silicon compounds such as silicon tetrachloride (SiCl.sub.4), trichlorosilane (SiHCl.sub.3) and silane (SiH.sub.4). The gaseous or liquid silicon compounds are then relatively readily purified by fractional distillation or like processes.
In the next step of preparing elemental silicon of solar grade purity, the purified silicon compound is reacted in gaseous phase to yield elemental silicon and usually a gaseous by-product. For example, silane gas is thermally decomposed in accordance with Equation I to yield silicon and hydrogen gas. ##STR1##
The above-summarized processes have, hitherto, been performed in the prior art to yield solid elemental silicon. Often the processes yield very low overall-density agglomerated particles of silicon, which are hard to handle in an efficient and continuous manner. Other examples of problems associated with the gas-to-solid thermal reaction processes are: undesirable deposition of a hard silicon crust on the reactor walls, and frequent interruption of the process due to the above-noted and other problems. For example, in accordance with the most widely used prior art modified Siemens process for chemical preparation of solar grade silicon, elemental silicon is grown epitaxially on the surface of rods disposed in a reactor wherein trichlorosilane (SiHCl.sub.3) and hydrogen (H.sub.2) gases are reacted. However, even this process must be interrupted from time to time in order to remove the solid silicon deposited on the rods, and to clean the reactor.
Another significant disadvantage of the prior art chemical processes for the preparation of solid silicon is that the resulting product is usually not sufficiently large grain crystalline to be directly suitable for semiconductor or solar cell applications. Therefore, the solid silicon produced by the prior art processes must be melted in a separate step and converted in a Czochralski or like crystal pulling apparatus into large grain crystalline (ideally monocrystalline) ingots, ribbons and the like. Thus, as is well appreciated by those skilled in the art, the overall prior art processes for preparing silicon solar cells require an undesirably high input of energy.
In addition to impurities, the new processes have another formidable problem; namely, how to limit or control the unwanted gas phase production of submicron silicon particles which are characteristic of the thermal decomposition, or pyrolysis, of silicon compounds, especially silane. Two reactions occur in the pyrolysis of a silicon hydride or halide:
(a) homogeneous decomposition reaction to produce fine powder of average particle size of about 0.1 micron; and PA1 (b) heterogeneous decomposition on solid surfaces to produce chemical vapor deposition (CVD) silicon with a metallic appearance.
Fine powder problems have delayed the development of various silicon processes using silane.
In order to overcome or alleviate the above-noted problems, a few attempts were made in the prior art to obtain molten, rather than solid, silicon in the thermal reaction process. For example, Japanese patent application laid open for public inspection on Dec. 2, 1977, Ser. No. 52-144959, describes a process wherein a bath of molten silicon (obtained from previously-prepared solid silicon of high purity) is maintained in a reaction vessel wherein trichlorosilane (SiHCl.sub.3) or silicon tetrachloride (SiCl.sub.4) and hydrogen gas (H.sub.2) are reacted. The silicon tetrachloride (SiCl.sub.4) or trichlorosilane (SiHCl.sub.3) is heated to 300.degree.-500.degree. C., and the hydrogen gas (H.sub.2) is heated to 1200.degree.-1600.degree. C. prior to introduction into the reaction vessel. The temperature is maintained in the gas containing part of the reaction vessel between 1050.degree. to 1150.degree. C. so that solid elemental silicon is formed in the vessel by the reaction of the gases. The solid silicon, however, falls into the bath of molten silicon where it melts.
A readily apparent disadvantage of the just-described process is that it is not suitable for production of silicon from silane (SiH.sub.4), because silane would already start significant thermal decomposition while being pre-heated prior to introduction into the reaction vessel. Furthermore, the reactants used in the process provide elemental silicon only in a relatively low yield. Still further, the process is batchwise, rather than continuous, in the sense that the gaseous reactants must be allowed to dwell in the reactor for a relatively long time to reach equilibrium. Perhaps for these and other reasons, according to the best knowledge of the present inventor, this prior art process has not gained even moderate industrial acceptance.
U.S. patent application Ser. No. 126,063 filed on Feb. 29, 1980, now U.S. Pat. No. 4,343,772, represents an attempt for production of molten silicon in a continuously operating reactor by thermal reaction of a suitable silicon containing precursor gas. In accordance with this disclosure, a precursor gas, such as silane, flows in an outer, forwardly moving vortex in a spiral flow reactor. A by-product gas, such as hydrogen, moves in an inner, rearwardly moving vortex. The walls of the reactor are maintained at a temperature above the melting point of silicon. Molten silicon flows downwardly on the walls of the reactor to collect in a pool wherefrom it is removed. A cooled injector probe having an internal diameter of about 0.06 inches is utilized to introduce the precursor gas tangentially relative to the interior cylindrical surface of the reactor. A vortex finder tube is disposed substantially in the center on the top of the reactor to remove the rearwardly moving vortex of the by-product gas.
The reactor described in the above-noted patent application, although designed to operate continuously for the production of molten silicon, is far from free of problems. More specifically, the injector tube is subject to frequent clogging due to formation of a solid silicon plug therein, and the emitted by-product gas contains a relatively large quantity of finely dispersed solid silicon particles.
The operation described in U.S. Pat. No. 4,343,772 results in thermal precipitation of silicon powder onto the probe inside the reactor. A nonmetal solid coating forms thereon and builds outwardly from the cooled probe, getting warmer as it extends. Eventually, the tip of the coating grows far enough from the probe so that its surface melts and wets. Thereafter, a capillary action phenomenon pulls liquid silicon back toward the cooled probe whereupon more solidifcation occurs, eventually resulting in plugging of the relatively small probe orifice. The significance of conversion of gas to solid prior to melting of silicon was unrecognized.
The location of the "vortex finder" near the top of the reactor and near the silane injector probe in U.S. Pat. No. 4,343,772, has contributed heavily to the loss of fines by delivering fine brown powder silicon out of the system. Also, the off center entrance of the cold silane probe into the extremely hot reactor caused large thermal stresses which resulted in cracking and sealing problems. The process of U.S. Pat. No. 4,343,772 limits silicon production because the gas entry and gas take-off are both at the top of the reactor. In cyclones of this design, there is an optimum length for centrifugation, as dictated by entering conditions and reversal of flow in going from outer vortex to inner vortex. Additional length loses the ability to produce additional effective cyclonic turns and is non-productive. The velocity of the gas entering the reactor in U.S. Pat. No. 4,343,772 is usually in the hundreds of feet per second.
Additional disclosures relating to the preparation of solar grade silicon are found in the following printed publications: Low-Cost Solar Array Project 5101-87, "Silicon Formation by Pyrolysis of Silane," Interim Report of the Continuous Flow Pyrolyzer Study, by H. Levin, Jet Propulsion Laboratory, California Institute of Technology, October 1978, and "Compatibility Studies of Various Refractory Materials in Contact with Molten Silicon," by O'Donnell et al., Jet Propulsion Laboratory, California Institute of Technology, March 1978 (JPL Publication 78-18).
In light of the foregoing, there still is a serious want in the prior art for a continuously operable efficient process and apparatus for chemically preparing molten silicon from a gaseous starting material. The present invention provides such a process and apparatus.